There is no doubt that Manchester city is winning two trophies this season just like you mentioned above. They are the strongest in FA Cup finals as Manchester united will not be able to withstand City in that match. We know how poor United performance is and that makes me give the cup to City. After all, when City clashed with United in the finals last season, City won them and the same thing will repeat itself again.
The better option for Manchester United if they are to have a chance in that game is for the coach to make use of young players like Diallo, Mainoo with a mix of the experience of Bruno Fernandes in their midfield for creative opportunities. He should also play Casemiro in his original position now that Lisandro Martinez is back because Casemiro has struggled with adapting to the role central back. If Ten Hag wins the FA, there is no doubt he will remain as coach of the team.